Optical system for increasing the contrast of pulsed laser radiation, laser system and method for increasing the contrast of pulsed laser radiation

ABSTRACT

An optical system for increasing contrast of pulsed laser radiation includes a first polarization setting optical unit for setting an elliptical polarization state of the pulsed laser radiation, and a multipass cell having at least two opposing mirrors. The pulsed laser radiation passes the multipass cell with formation of a plurality of intermediate focus zones. The multipass cell is filled with a gas having an optical nonlinearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation, such that the multipass cell outputs beam portions having differently aligned elliptical polarization states on account of the intensity-dependent rotation. The optical system further includes an optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/EP2021/054863 (WO 2021/170815 A1), filed on Feb. 26, 2021, and claims benefit to German Patent Application No. DE 10 2020 105 015.1, filed on Feb. 26, 2020. The aforementioned applications are hereby incorporated by reference herein.

FIELD

Embodiments of the present invention relate to optical systems for increasing the contrast of pulsed laser radiation and to laser systems, in particular ultrashort pulse (USP) laser systems, for emitting pulsed laser radiation with high power and/or high pulse energy. Furthermore, embodiments of the present invention relate to a method for increasing the contrast of pulsed laser radiation, in particular of ultrashort pulse trains.

BACKGROUND

When pulsed laser radiation is used, the contrast between primary laser pulses having a high peak intensity and the background is a significant parameter. The laser radiation that forms the background usually consists of a radiation pedestal attributable to ASE, and/or of laser pulses which arrive temporally before or after the primary laser pulses and the peak intensity of which is significantly lower than the peak intensity of the primary laser pulses.

In order to reduce an interaction of the background during an experiment or during material processing, for example, an increase in contrast is usually carried out. The aim here is to largely remove the ASE background and the laser pre- and postpulses from the pulsed laser radiation and thus to largely restrict the intensity contribution for the interaction to the primary laser pulses.

The prior art discloses contrast enhancement methods in which a laser beam is guided in a gas-filled hollow core fiber (HCF) in order for example to cause a rotation of an elliptical polarization on account of nonlinear effects. This is known as Nonlinear Elliptical polarization Rotation (also called Nonlinear Ellipse Rotation, NER). In the HCF, the NER is accompanied by a nonlinear spectral broadening of the intensive laser pulses. In particular, a constant gas condition can be provided along the HCF. Alternatively, by way of differential pumping, the gas density can decrease along the fiber in order to prevent unwanted nonlinear effects or ionization from arising. The use of NER in conjunction with an HCF is disclosed for example in “Generation of high-fidelity few-cycle pulses via nonlinear ellipse rotation in stretched hollow-fiber compressor” by N. Khodakovskij, CLEO 2018, OSA 2018. Furthermore, in “Contrast improvement of sub-4 fs laser pulses using nonlinear elliptical polarization rotation” by N. Smijesh et al., Optics Letters, Vol. 44, No. 16, Aug. 15, 2019, NER is used to improve the contrast of sub-4 fs laser pulses using an HCF filled with argon gas.

Furthermore, DE 10 2014 007159 A1 discloses a method for the spectral broadening of laser pulses for nonlinear pulse compression using a multipass cell constructed in the form of a so-called Herriott cell, for example. In that case, the aim is a spectral broadening of laser pulses which can be carried out even in the case of a pulse power which is greater than the critical power of the nonlinear medium used for the spectral broadening.

SUMMARY

Embodiments of the present invention provide an optical system for increasing contrast of pulsed laser radiation. The optical system includes a first polarization setting optical unit for setting an elliptical polarization state of the pulsed laser radiation, and a multipass cell having at least two opposing mirrors. The pulsed laser radiation passes the multipass cell with formation of a plurality of intermediate focus zones. The multipass cell is filled with a gas having an optical nonlinearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation, such that the multipass cell outputs beam portions having differently aligned elliptical polarization states on account of the intensity-dependent rotation. The optical system further includes an optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states.

BRIEF DESCRIPTION OF THE DRAWINGS

Subject matter of the present disclosure will be described in even greater detail below based on the exemplary figures. All features described and/or illustrated herein can be used alone or combined in different combinations. The features and advantages of various embodiments will become apparent by reading the following detailed description with reference to the attached drawings, which illustrate the following:

FIG. 1 shows an exemplary schematic illustration of a laser system comprising an optical system for contrast enhancement according to embodiments;

FIGS. 2A, 2B, and 2C show exemplary schematic diagrams for elucidating a Herriott cell as an example of a multipass cell according to embodiments; and

FIG. 3 shows a schematic flow diagram for elucidating an exemplary procedure for contrast enhancement according to embodiments.

DETAILED DESCRIPTION

Embodiments of the present invention provide systems and methods which can be used for increasing the contrast of pulsed laser radiation, for example of ultrashort pulse trains, even and in particular in the case of high pulse energies and high average powers. In particular, embodiments of the present intention utilize nonlinear effects for influencing the polarization of laser pulses and the propagation thereof.

In one aspect, an optical system for increasing the contrast of pulsed laser radiation using a nonlinear elliptical polarization rotation comprises:

a first polarization setting optical unit for setting an elliptical polarization state of the pulsed laser radiation,

a multipass cell having two opposing mirrors forming in particular a concentric or confocal resonator, wherein the pulsed laser radiation repeatedly passes through the multipass cell, in particular the resonator, with formation of a plurality of intermediate focus zones, wherein the multipass cell is filled with a gas having an optical nonlinearity which causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation, such that the multipass cell outputs beam portions having differently aligned elliptical polarization states on account of the intensity-dependent rotation, and

an optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states.

In a further aspect, a laser system, in particular an ultrashort pulse (USP) laser system, for emitting pulsed laser radiation (9) comprises

a laser radiation source, which outputs pulsed laser radiation comprising primary laser pulses, in particular having pulse energies and pulse durations in the range of a few hundred femtoseconds or less,

optionally a pulse duration setting system for setting the pulse duration of the primary laser pulses,

at least one optical system as disclosed herein for increasing the contrast of the pulsed laser radiation using a nonlinear elliptical polarization rotation in a plurality of intermediate focus zones of a multipass cell. Optionally, the laser system can comprise an optical pulse duration compressor system for compensating for a dispersive contribution of the at least one optical system and optionally for temporally compressing the primary laser pulses of the laser radiation if they have experienced a nonlinear spectral broadening in at least one of the intermediate focus zones.

In a further aspect, a method for increasing the contrast of pulsed laser radiation using a nonlinear elliptical polarization rotation comprises the following steps:

setting an elliptical polarization state of the pulsed laser radiation,

input coupling the pulsed laser radiation into a multipass cell having two mirrors forming in particular a concentric or confocal resonator, wherein the multipass cell, in particular the resonator, is repeatedly traversed with formation of a plurality of intermediate focus zones, wherein the multipass cell is filled with a gas having an optical nonlinearity which causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation in the intermediate focus zones, as a result of which beam portions having differently aligned elliptical polarization states are generated in the multipass cell,

output coupling the beam portions having differently aligned elliptical polarizations out of the multipass cell, and

separating the output-coupled beam portions into a useful beam portion with primary laser pulses and a residual beam portion with low-intensity laser radiation.

In some developments of the optical system, for a rotation of the alignment of one of the elliptical polarization states, in particular for setting a rotation angle of 90°, at least one of the following parameters of the optical system can be set or settable:

a gas pressure in the intermediate focus zones, wherein the optical system is embodied as a gas-filled cell and the optical system has a pressure setting device for setting the gas pressure in the gas-filled cell, and/or

a dispersion present in the multipass cell.

Optionally, furthermore, at least one of the following parameters of the optical system can be set or settable:

an ellipticity of the set elliptical polarization of the pulsed laser radiation,

a number of intermediate focus zones in the multipass cell,

focus diameters in the intermediate focus zones, and

Rayleigh lengths of the intermediate focus zones.

In some developments, the first polarization setting optical unit comprises a first waveplate, optionally a λ/4 waveplate and/or a λ/2 waveplate.

In some developments, the optical system can furthermore comprise at least one of the following optical components:

a pulse duration setting system for setting a pulse duration of primary laser pulses of the pulsed laser radiation,

a first optical telescope arrangement, which is set to image the pulsed laser radiation onto a predefined mode in the multipass cell and which is optionally arranged downstream of the first polarization setting optical unit,

an input coupling mirror for coupling the pulsed laser radiation into the multipass cell,

an output coupling mirror for forwarding the pulsed laser radiation emerging from the multipass cell, and

a second optical telescope arrangement, which is set to collimate the pulsed laser radiation emerging from the multipass cell.

In some developments of the optical system, the multipass cell can be embodied:

with a predetermined or settable number of intermediate focus zones, which are produced with the opposing mirrors for example in a resonator set-up with, in particular identical, radii of curvature of the mirrors, wherein the intermediate focus zones have substantially an identical diameter and an identical Rayleigh length,

for shaping intermediate focus zones arranged next to one another and optionally so as to be partly superposed on one another,

as a cell filled with a nonlinear gaseous medium, in particular a noble gas such as helium or argon, wherein the same gas pressure is present in each of the intermediate focus zones,

with, present in the intermediate focus zones, a substantially constant pulse duration and a substantially constant pulse energy of the pulsed laser radiation passing through, and/or

for stepwise nonlinear spectral broadening of the pulsed laser radiation passing through in the intermediate focus zones.

In some developments of the optical system, at least one of the mirrors of the multipass cell can be embodied as a convex mirror, wherein the radii of curvature match, in particular, and/or a distance between the mirrors lies in a range of 95% to 105% of the sum of the radii of curvature. Alternatively or additionally, at least one of the mirrors can be embodied as a dispersive mirror, the dispersion contribution of which compensates for a dispersive contribution of at least one pass of a primary laser pulse of the pulsed laser radiation through the multipass cell. Alternatively or additionally, at least one of the mirrors can comprise at least one mirror segment on which the pulsed laser radiation impinges at least once during the circulation of the pulsed laser radiation through the multipass cell.

In some developments of the optical system, the multipass cell can be embodied in such a way that a primary laser pulse of the pulsed laser radiation, the contrast of which pulse is intended to be increased in the optical system, experiences substantially no change in the pulse duration and/or pulse energy in the intermediate focus zones.

In some developments of the optical system, the beam portions having differently aligned elliptical polarization states can comprise a useful beam portion with primary laser pulses and a residual beam portion with low-intensity laser radiation. The residual beam portion forms a background composed of a radiation pedestal attributable to ASE, and/or of laser pulses which arrive temporally before or after the primary laser pulses and the peak intensity of which is significantly lower than the peak intensity of the primary laser pulses. In this regard, the radiation background can have in particular low-intensity laser pulses preceding the high-intensity laser pulses and/or low-intensity laser pulses succeeding said high-intensity laser pulses.

In some developments of the optical system, the optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states can comprise:

a second polarization setting optical unit for returning each of the differently aligned elliptical polarization states to a linear polarization state, wherein in particular the beam portions having differently aligned elliptical polarization states that are output by the multipass cell are converted into a useful beam portion and a residual beam portion having differently aligned linear polarization states, and

a beam splitter, which outputs the useful beam portion and the residual beam portion on different beam paths.

In particular, the second polarization setting optical unit can comprise a second waveplate, optionally a λ/4 waveplate and/or a λ/2 waveplate.

In some developments, the optical system can furthermore comprise a control system, which is configured for rotating the alignment of one of the elliptical polarization states, in particular for setting a rotation angle of 90°, at least for setting one of the following parameters:

a pulse duration of primary laser pulses of the pulsed laser radiation,

a pulse energy of primary laser pulses of the pulsed laser radiation,

an ellipticity of the set elliptical polarization of the pulsed laser radiation,

focus diameters in the intermediate focus zones,

Rayleigh lengths of the intermediate focus zones, and

a gas pressure of the gas in the intermediate focus zones.

In some developments of the laser system, gas conditions, in particular gas type and/or gas pressure, in a first optical system can differ from corresponding gas conditions in a second optical system.

In some developments of the method, the latter can furthermore comprise the following step:

setting at least one of the following parameters for rotating the alignment of one of the elliptical polarization states in the multipass cell, in particular for setting a rotation angle of 90° for primary laser pulses of the pulsed laser radiation:

a gas pressure in the intermediate focus zones and/or

a dispersion of the primary laser pulses that has accumulated in the multipass cell.

Optionally, the following can furthermore be set:

a pulse duration of the primary laser pulses of the pulsed laser radiation,

a pulse energy of the primary laser pulses of the pulsed laser radiation,

an ellipticity of the set elliptical polarization state of the pulsed laser radiation,

focus diameters in the intermediate focus zones, and/or

Rayleigh lengths of the intermediate focus zones.

In some developments of the method, furthermore a pulse spectrum of the pulsed laser radiation, said pulse spectrum being assigned to the primary pulses, can broaden, in particular from intermediate focus zone to intermediate focus zone, on account of a nonlinear spectral broadening in the multipass cell, while the increase in contrast simultaneously takes place.

Embodiments of the present invention use the approach of NER using a gas-filled multipass cell, for example a Herriott cell, for enhancing the contrast of pulsed laser radiation, wherein this can optionally be effected together with a spectral pulse broadening and a subsequent pulse duration compression. The concepts proposed herein have the advantages that the increase in the pulse contrast can be carried out in the case of high average powers (greater than a few 100 W into the kW range) and very high pulse energies (into the J range) and can be implemented in an easily realized and very efficient way.

In the context of the approach of NER using a gas-filled multipass cell as considered herein, an elliptical polarization state is understood to mean a polarization state in which a polarization ellipse is present, such as can be caused for example by setting an angle in a range of between 0° and 45° or in a range of between 45° and 90° (analogously in a range of between 45° and 135° or in a range of between 135° and 180°) of a fast axis of a λ/4 waveplate and a polarization plane of an incident linearly polarized laser beam. A polarization ellipse of the electric field forms in these angle ranges. In one exemplary elliptical polarization state, 90% of the laser beam can be present in an s-polarization state and 10% in a p-polarization state. An elliptical polarization state thus differs from a linear polarization state (having only one portion in the s- or p-polarization state) and also from a circular polarization state (angle of 45° of the λ/4 waveplate or identical portions in the s-polarization state and in the p-polarization state).

Aspects described herein are based in part on the insight that a nonlinearly intensity-dependent rotation of an alignment of an elliptical polarization can be caused on account of an intensity-dependent phase rotation (self-induced ellipse rotation). The intensity dependence of the elliptical polarization rotation has the effect that the polarization of a pulse experiences a different rotation near the peak of the pulse compared with the low-intensity regions. As a result, it becomes possible to separate high-intensity beam portions (formed by the primary laser pulses provided for an application) from low-intensity laser radiation (for example formed by the pre- and postpulses accompanying the primary laser pulses). The implementation of NER in the context of a multipass cell provides diverse setting parameters and control parameters, thereby enabling contrast enhancement in an optical system for a wide variety of parameter configurations of a laser system.

Furthermore, the inventors have recognized that with the aid of the concepts disclosed herein concerning NER in the case of use in a Herriott cell, pulsed laser radiation can be freed of low-energy laser radiation (e.g. laser pre- or postpulses) by virtue of only laser pulses having a high intensity (the primary laser pulses discussed) being influenced in terms of their polarization with regard to the downstream beam path. The influencing is effected by way of the nonlinearity of the refractive index of the gas in intermediate focus zones of the Herriott cell in such a way that the primary laser pulses can pass through an optical beam splitting system without any loss if possible. In contrast to the primary laser pulses, the alignment of the polarization ellipse of the low-energy laser radiation portion does not change within the Herriott cell, and so said low-energy laser radiation portion can be removed from the laser radiation after leaving the Herriott cell by means of the optical beam splitting system. In other words, laser pre- and/or postpulses can be removed from the laser radiation if the rotation of the polarization ellipse is set only for the primary laser pulses on account of the high intensity thereof in such a way that only the primary laser pulses can pass through a preset beam splitter, for example.

FIG. 1 shows a laser system 1 comprising an optical system 3 for increasing the contrast. The optical system 3 is based on the use of a multipass cell 5 filled with gas 4 (for example a Herriott cell), the gas 4 serving as a nonlinear Kerr medium. By way of example, helium can be used in the case of very high intensities in the multipass cell 5. In the case of lower but still high intensities in the multipass cell 5, for example argon or some other noble gas can be used as a nonlinear medium.

The laser system 1 generally comprises a laser radiation source 7, which outputs laser radiation 9. The laser radiation 9 comprises primary laser pulses 11 having a pulse energy in the range of e.g. a few hundred millijoules and a pulse duration Δt in the range of a few hundred femtoseconds or less, which form an ultrashort pulse train, for example. Depending on the laser radiation source 7, the laser radiation 9 furthermore comprises low-energy laser radiation 13, indicated by way of example as prepulses 13A or postpulses 13B in FIG. 1 . Furthermore, the laser radiation source 7 can optionally have a pulse duration setting system 15 for setting the pulse duration of the primary laser pulses 11, wherein the pulse duration setting system 15 can also be assigned to the optical system 3, as indicated in FIG. 1 .

It is assumed in the present example that at the output of the pulse duration setting system 15 or at the output of the laser radiation source 7, there is present the laser radiation 9 having a linear polarization 17A, the polarization vector of which is indicated as orthogonal to the plane of the drawing by way of example in FIG. 1 . That is to say that both the primary laser pulses 11 and the low-energy laser radiation 13 are linearly polarized, specifically generally in the same direction.

For increasing the contrast, the optical system 3 has a first polarization setting optical unit 19. In the latter, the laser radiation 9 is elliptically polarized. In this respect, FIG. 1 indicates at the output of the first polarization setting optical unit 19 an electric field vector circulating on a polarization ellipse in order to clarify an elliptical polarization state 17B by way of example, the semimajor axis of the polarization ellipse running in the plane of the drawing by way of example. For the purpose of setting this elliptical polarization state of the laser radiation 9, the first polarization setting optical unit 19 comprises a waveplate (e.g. a λ/4 waveplate or a λ/8 waveplate, etc.). In the example in FIG. 1 , the first polarization setting optical unit 19 comprises a first λ/4 waveplate 19A. Furthermore, the first polarization setting optical unit 19 can optionally have, for example upstream of the first λ/4 waveplate 19A, a λ/2 waveplate 19B for aligning the orientation of the elliptical polarization state. For this purpose, the setting of the polarization in the first polarization setting optical unit 19 is independent of the intensity since the waveplates operate with an anisotropic refractive index, for example with a birefringent crystal. For the purpose of setting the elliptical polarization state of the laser radiation 9, for example, the λ/4 waveplate is set in relation to the polarization plane of the laser radiation 9 in such a way that an angle between a fast axis of the λ/4 waveplate and the polarization plane is not 0°, 45°, 90° or 135°. Corresponding angle ranges can be assigned to other waveplates or combinations of waveplates for producing elliptical polarization. Setting the angle of the waveplate makes it possible to set an ellipticity of the polarization ellipse. In combination with the upstream λ/2 waveplate 19B, an alignment of the polarization ellipse can furthermore be effected.

On account of the linear polarization present identically for the primary laser pulses 11 and the low-energy laser radiation 13 at the input of the first polarization setting optical unit 19, these two beam portions also have the same elliptical polarization state 17B upon leaving the first polarization setting optical unit 19.

FIG. 1 furthermore shows a telescope arrangement 21 for matching the mode (generally beam parameters such as beam diameter and beam divergence) of the pulsed laser radiation 9 prior to input coupling into the multipass cell 5 by means of an input coupling mirror 23.

The multipass cell 5 embodied as a Herriott cell, for example, comprises two concavely curved mirrors 25A, 25B, which form a beam path 5A running back and forth repeatedly between the mirrors 25A, 25B in a gas-filled environment.

The multipass cell 5—for example in the geometry of a Herriott cell—can be formed by two concave mirrors aligned with one another for example in a concentric or confocal resonator arrangement, generally also in any other resonator configuration, along a common optical axis 27 (given by the concentric arrangement). In this case, the mirrors 25A, 25B are also referred to as Herriott mirrors. If the laser radiation 9 is introduced into the multipass cell 5 in a manner offset with respect to the optical axis 27, the laser radiation 9 will circulate repeatedly there and back on a predefined, usually elliptical (circular) pattern.

FIG. 2A schematically illustrates the beam path between the mirrors 25A, 25B with the formation of an intermediate focus zone 29, presupposing a correspondingly matched mode of the input-coupled laser radiation 9. The intermediate focus zone 29 has for example a focus diameter d and a Rayleigh length Lr and lies in the region of a plane of symmetry 31 of the resonator arrangement, embodied in a concentric fashion in the example in FIG. 1 .

FIGS. 2B and 2C show plan views of the mirrors 25A, 25B, in which circularly arranged impingement regions 33 on the mirror surfaces are indicated schematically. The laser radiation 9 impinges as centrally as possible in the impingement regions 33 before it is reflected back from there again in the direction of the center of the multipass cell (here the resonator arrangement). FIGS. 2B and 2C furthermore reveal an input coupling opening 35A and also an output coupling opening 35B. The regions available for the reflection on the surface of the mirrors 25A, 25B are circular area sections having a diameter D. The number of circulations (intermediate focus zones 29) can be of arbitrary magnitude, in principle; for example, 5 to 100 intermediate focus zones can be traversed; that is to say that a plurality of intermediate focus zones are traversed in the multipass cell. Furthermore, at least one of the mirrors 25A, 25B can also be constructed from individual discrete mirror elements, wherein a reflection can preferably take place on one individual mirror element. For example, twelve intermediate focus zones 29 are traversed.

As an alternative to the beam input coupling and beam output coupling through openings in the mirrors, smaller mirror elements engaging in the Herriott cell can be used and positioned at the positions of the openings 35A, 35B, for example.

Referring to the beam path 5A indicated in FIG. 1 , the pulsed laser radiation 9 is repeatedly guided through intermediate focus zones in the center of the multipass cell 5. High intensities form in the intermediate focus zones on account of the focusing of the primary laser pulses during the pulse duration Δt of the primary laser pulses 11, and result in a nonlinear behavior of the refractive index of the gas 4. As explained below, the nonlinear behavior of the refractive index of the gas 4 results in beam portions of the laser radiation 9 having different polarization states and can thus be used for increasing the contrast of the pulsed laser radiation 9.

After the predetermined number of passes through the multipass cell 5, the laser radiation 9 leaves the multipass cell 5 and impinges on an output mirror 37, which reflects the output-coupled laser radiation. The output mirror 37 directs the laser radiation 9 through a second telescope arrangement 39, which recollimates the laser radiation 9.

For the purpose of splitting the beam portions of the laser radiation 9 having different polarization states, the optical system 3 furthermore has an optical beam splitting system 41. In the embodiment shown by way of example in FIG. 1 , the optical beam splitting system 41 comprises a second waveplate 43 and a beam splitter 45. The second waveplate 43 is generally set in such a way that the elliptical polarization state is converted into a linear polarization again. Further set-ups for splitting beam portions having different elliptical polarization states are known from the prior art.

On account of the nonlinearity of the refractive index n of the gaseous Kerr medium in the multipass cell 5—i.e. for an intensity-dependent refractive index n=n_0+n_2*I(r; t) with the gas-specific refractive index parameters n_0 and n_2 and the intensity profile I(r; t) in the intermediate focus zone—a rotation of the polarization ellipse by a rotation angle ΔΘ arises only for the primary laser pulses 11, wherein the rotation angle ΔΘ is proportional to the intensity I(r; t) and n_2. A polarization ellipse rotated by ΔΘ is indicated by way of example as an elliptical polarization state 17C in FIG. 1 .

The orientation of the polarization ellipse of the low-energy laser radiation 13 results from the ordinary refractive index of the gas 4 and remains substantially unchanged. The elliptical polarization states of the primary laser pulses 11 and of the low-energy laser radiation 13 thus change their relative alignment with respect to one another upon every pass through an intermediate focus zone 29.

If the alignments of the two polarization ellipses downstream of the multipass cell 5 differ by 90°, for example, after traversal of the waveplate 43 this results in two linear polarization states 47A, 47B aligned orthogonally to one another for the primary laser pulses 11 and the low-intensity laser radiation 13. If the beam splitter 45 is correspondingly aligned, the beam splitter 45 distributes the primary laser pulses 11 and the low-intensity laser radiation 13 between two different beam paths for a useful beam portion 9A and a residual beam portion 9B, which correspond to the orthogonal linear polarization states 47A, 47B.

FIG. 1 indicates by way of example a primary laser pulse 11′ of the useful beam portion 9A, from which pre- and postpulses have been removed. The pre- and postpulses form the residual beam portion. Even if the resulting beam portions are not polarized fully orthogonally to one another, significant portions of the pre- or postpulses can be removed from the useful beam path.

For the subsequent use of the primary laser pulses 11′, the contrast of which has been increased in relation to the low-intensity laser radiation, the primary laser pulses 11′ can be added to a compressor 49, for example. The compressor 49 is illustrated by way of example as a chirped mirror compressor in FIG. 1 . A useful laser radiation 9A′ comprising a train of compressed primary laser pulses 11″ can thus be output at an output of the laser system 1, the compressed primary laser pulses 11″ having an increased contrast.

In contrast to the set-up known from the prior art that uses an HCF, the configuration proposed herein e.g. using a Herriott cell can enable a predetermined/settable number of intermediate focus zones 29 to be traversed. Moreover, a beam diameter d in the intermediate focus zones is settable and can for example also be coordinated with the laser power, pulse duration, etc., and the gas 4 by way of the radius Rm of curvature of the mirrors 25A, 25B. The radius Rm of curvature is identical for both mirrors, for example, or is at least of the same order of magnitude.

Besides a settability of the size of the intermediate focus zone 29 e.g. by way of the radii of curvature of the mirrors 25A, 25B and also by way of a corresponding telescope arrangement for mode matching, which can be disposed upstream of the multipass cell, the gas pressure can be set in regard to the nonlinearity. It is noted that on account of the high spatial proximity of the various intermediate focus zones being traversed in the multipass cell, the same gas pressure is present in each of the intermediate focus zones. Preferably, the optical beam parameters and beam properties in the various intermediate focus zones are very similar, with the result that similar nonlinear effects are present as well.

If the mirrors 25A, 25B form a concentric resonator (distance between mirrors approximately 2*Rm given identical radii Rm of curvature), the intermediate focus zones 29 substantially all have the same diameter d and have correspondingly identical Rayleigh lengths Lr. Generally, the distance between the mirrors 25A, 25B lies in a range of 95% to 105% of the sum of the radii of curvature. An intensive primary laser pulse 11 propagates through these intermediate focus zones 29 sequentially and in the process interacts repeatedly with the gas 4 at electric field strengths which can bring about nonlinear effects on the refractive index n and thus on the polarization state of the primary laser pulse 11.

The use of the Herriott set-up described herein provides various parameters that are definable in advance and/or settable during operation for the design of the intermediate focus zones and the nonlinear conditions present therein. For setting the parameters, the optical system 3 can comprise a control system 61, for example, which is connected via control connections 63 to the pulse duration setting system 15, the polarization setting optical unit 19 (in particular for setting the angular positions of the first λ/4 waveplate 19A and optionally the λ/2 waveplate 19B), the telescope arrangements 21, 39 (in particular for setting the distance between telescope lenses 21A, 21B), a pressure setting device 65 for setting the gas pressure (see FIG. 1 ) and/or the optical beam splitting system 41 (in particular for setting the angular position of the second λ/4 waveplate 43).

The following can be set, for example, with the aid of the control system 3:

the pulse duration Δt of the primary laser pulses 11 of the pulsed laser radiation 9,

the pulse energy of the primary laser pulses 11 of the pulsed laser radiation 9,

an ellipticity of the set elliptical polarization of the pulsed laser radiation 9,

the focus diameters d in the intermediate focus zones 29,

the Rayleigh lengths Lr of the intermediate focus zones 29, and

a gas pressure of the gas 4 in the intermediate focus zones 29.

As shown in FIG. 2B and FIG. 2C, the laser radiation 9 repeatedly impinges on the mirrors 25A, 25B (a number of times in each case). The mirrors can supplementarily be used for dispersion matching by virtue of their being embodied as dispersive mirrors. If the mirrors 25A, 25B have a dispersive effect at least in the case of one of the reflections, it is possible to directly affect the dispersion and thus the pulse duration of the primary laser pulses 11. By way of example, one or more of the impingement regions 33 can be provided with a dispersive layer. A dispersive coating 51 is indicated in a dashed manner for the mirror 25B by way of example in FIG. 2A. Moreover, each of the mirrors 25A, 25B can be constructed from a plurality of mirror segments having predetermined dispersive properties, the dispersion of each of the mirror segments being matched to a desired pulse duration in the pass through the multipass cell 5. Accordingly, the dispersion present in the multipass cell 5 is composed of a dispersion contribution of the dispersive mirrors and a dispersion contribution in the gas-filled volume along the beam path 5A.

One exemplary mirror segment 53 is indicated in FIG. 1 . During the circulation of the pulsed laser radiation through the multipass cell, the laser radiation impinges (depending on the size of the mirror segment) at least once on the mirror segment, which usually has at least an extent of the magnitude of the beam diameter D on the mirror surface.

In other words, the concepts proposed herein allow a dispersion accumulated during passage through the gas-filled volume to be at least partly compensated for by suitable dispersive mirror coatings (chirped mirrors) in order for example to maintain comparable pulse durations in the intermediate focus zones or to vary the pulse durations in a targeted manner.

In addition to the use of NER discussed hereinabove, the multipass cell 5 enables the high-intensity primary laser pulses to be spectrally broadened stepwise in the intermediate focus zones. The nonlinear spectral broadening is brought about by the high intensity present in each case in an intermediate focus zone and by the nonlinearity of the refractive index of the gaseous medium in the multipass cell 5.

On account of the nonlinear spectral broadening, the pulse spectrum can change from intermediate focus zone to intermediate focus zone, specifically substantially with a constant pulse duration and constant pulse energy. If the multipass cell 5 is constructed by means of chirped mirrors, the pulse duration can additionally be set. By way of example, the pulse duration can change (shorten or lengthen) from pass to pass. Accordingly, the peak intensities in the intermediate focus zones remain substantially constant even in the case of a nonlinear spectral broadening.

A further advantage can arise in connection with the nonlinear spectral broadening if laser radiation having elliptical polarization is used for this purpose in the multipass cell. In this regard, the spectral broadening can possibly be effected inherently more smoothly across the frequency spectrum, such that a less structured spectrum can arise. This can have a positive effect on the subsequent pulse shaping and/or pulse compression.

FIG. 2A shows the formation of an intermediate focus zone in a Herriott cell, assuming curved Herriott mirrors. Geometric parameters for the implementation of a multipass cell in the context of the concepts presented herein are considered below.

The limiting of the pulse energies of primary laser pulses which can be increased in terms of contrast (and optionally be spectrally broadened) by means of a multipass cell results from an avoidance of laser-induced damage to the (Herriott) mirrors and from the ionization threshold value of the gas used. With the use of helium as gas 4 in the multipass cell 5, a highest possible ionization threshold value is approximately 3.42 10{circumflex over ( )}14 W/cm2.

The laser-induced damage to the mirrors 25A, 25B dictates a minimum diameter of the laser radiation 9 on the curved mirrors 25A, 25B. The ionization threshold value determines the smallest possible focus diameter d in the intermediate focus zones 29 with regard to avoiding an ionization of the gas 4. Both parameters together define a required length of the multipass cell 5, i.e. the distance between the mirrors from which the concentric resonator is constructed, for example, and also the radius of curvature thereof.

With regard to the limiting by way of multiphoton ionization, it should be mentioned that the latter is substantially independent of the gas pressure in the multipass cell.

What is also of importance in this context is that the electric field strength required for an ionization is increased for circularly/elliptically polarized light in comparison with linearly polarized light, such that for a comparable beam diameter D on the mirrors 25A, 25B, the possible focus diameter d in the intermediate focus zones 29 can be chosen to be smaller. This generally results in a reduction of the distance between the mirrors (for example a distance between the mirrors 25A, 25B that is shortened by at least a factor of 2) in comparison with a multipass cell operated with linear polarization. Consequently, the use of a multipass cell for NER can have the positive side effect that the multipass cell can be constructed more compactly.

For the application of high-intensity laser radiation, it may be necessary for the mirrors to withstand pulse energies of a few 100 mJ in conjunction with pulse durations of a few 100 fs, for example 500 fs or shorter. For ultrashort pulses, the mirrors should furthermore be of wideband design, e.g. designed for a wavelength range of e.g. 700 nm to 1100 nm, for example, for ultrashort pulses from a titanium-sapphire laser or 900 nm to 1100 nm for ultrashort pulses from lasers that emit around 1000 nm, such as Nd:YAG or Yb:YAG. Furthermore, the mirrors may or may not provide a dispersion contribution, and so dispersive coatings should possibly also be taken into account.

Exemplary parameters for a multipass cell and the mirrors on which it is based are explained below with reference to FIGS. 2A to 2C. For coated mirrors, it is possible to measure a laser-induced damage threshold value of approximately 0.5 J/cm2 in the case of a pulse duration of approximately 500 fs. This threshold value is usually assigned to the beam center. Assuming a Gaussian beam, e.g. a threshold value of approximately 0.1 J/cm2 thus results for the approximately 500 fs laser pulse, and so given a safety factor of 3, for example, the maximum permissible fluence would be approximately 0.03 J/cm2.

What results on the basis of this is e.g. a beam radius of approximately 9 mm for 200 mJ pulses or a converted 1/e2 beam diameter of approximately 13 mm on the mirrors 25A, 25B for the multipass cell 5 with a required focal length f of the mirrors 25A, 25B (radius Rm of curvature) and correspondingly a length of the multipass cell 5 (distance between mirrors).

For circularly polarized light (having a (maximum) electric field strength that is reduced in comparison with linearly polarized light), it is possible to implement a reduced distance between the mirrors/a shortened multipass cell length/resonator length L with a correspondingly smaller radius of curvature of the mirrors 25A, 25B, assuming that the same beam diameter is present on the mirrors.

The steps in the procedure proposed herein for increasing the contrast of pulsed laser radiation using a nonlinear elliptical polarization rotation will be explained with reference to the flow diagram shown in FIG. 3 .

Step 71 involves setting an elliptical polarization state 17A of the pulsed laser radiation 9.

Step 73 involves input coupling the pulsed laser radiation 9 into a multipass cell 5 formed from two concave mirrors 25A, 25B, which form e.g. a concentric or confocal resonator. The multipass cell 5 (e.g. a concentric or confocal resonator) is repeatedly traversed with formation of a plurality of intermediate focus zones 29, wherein the multipass cell 5 is filled with a gas 4 having an optical nonlinearity which causes an intensity-dependent rotation of an alignment of the elliptical polarization state 17B of the pulsed laser radiation 9 in the intermediate focus zones 29. Accordingly, beam portions having differently aligned elliptical polarization states are generated in the multipass cell 5.

Step 75 involves output coupling the beam portions having differently aligned elliptical polarizations out of the multipass cell 5, and step 77 involves separating the output-coupled beam portions into a useful beam portion 9A with primary laser pulses 11′ and a residual beam portion 9B with low-intensity laser radiation.

Furthermore, in accordance with the procedure disclosed herein, optionally at least one of the following parameters for rotating the alignment of one of the elliptical polarization states in the multipass cell 5 can be set in a preceding or operation-accompanying step 79:

a pulse duration Δt of the primary laser pulses 11 of the pulsed laser radiation 9,

a pulse energy of the primary laser pulses 11 of the pulsed laser radiation 9,

an ellipticity of the set elliptical polarization state of the pulsed laser radiation 9,

focus diameters d in the intermediate focus zones 29,

Rayleigh lengths Lr of the intermediate focus zones 29,

a gas pressure in the intermediate focus zones 29 and

a dispersion of the primary laser pulses (11) that has accumulated in the multipass cell 5.

In this regard, it is possible to set for example a rotation angle ΔΘ of 90° for primary laser pulses 11 of the pulsed laser radiation 9.

In summary, it is possible to set the necessary nonlinear conditions for an NER for removing low-energy contributions from the laser radiation in a multipass cell embodied as a Herriott cell. As explained, the optical systems proposed herein allow for the possibility of providing substantially very similar (comparable) conditions of the nonlinearity in the intermediate focus zones of the multipass cell, for example on account of the comparable gas pressure and the comparable pulse durations and pulse energies (and thus pulse peak intensities) in the intermediate focus zones.

Moreover, the pulse durations and pulse peak intensities can furthermore be set by means of the abovementioned dispersive mirrors/mirror segments for the compensation of (often only small) pulse duration lengthening effects during the individual passes through the multipass cell. In particular, with the use of different dispersive mirror segments, it is possible to obtain additional freedoms for a dispersion compensation and thus for a setting of the intensity in the intermediate focus zones.

Furthermore, it is possible for the laser radiation to pass through a sequence of successive multipass cells one after another. This allows gas conditions to be set in a differentiated manner with respect to the groups of intermediate focus zones respectively present in the individual multipass cells.

It is explicitly emphasized that all features disclosed in the description and/or the claims should be regarded as separate and independent of one another for the purpose of the original disclosure and likewise for the purpose of restricting the claimed invention independently of the combinations of features in the embodiments and/or the claims. It is explicitly stated that all range indications or indications of groups of units disclose any possible intermediate value or subgroup of units for the purpose of the original disclosure and likewise for the purpose of restricting the claimed invention, in particular also as a limit of a range indication.

While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above.

The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.

LIST OF REFERENCE SIGNS

Laser system 1

Optical system 3

Gas 4

Multipass cell 5

Beam path 5A

Laser radiation source 7

Laser radiation 9, 9A, 9A′, 9B

Primary laser pulses 11, 11′, 11″

Low-energy laser radiation 13

Prepulse 13A

Postpulse 13B

Pulse duration setting system 15

Linear polarization 17A

Elliptical polarization state 17B, 17C

Polarization setting optical unit 19

First λ/4 waveplate 19A

λ/2 waveplate 19B

Telescope arrangement 21

Telescope lenses 21A, 21B

Input coupling mirror 23

(Herriott) mirrors 25A, 25B

Optical axis 27

Intermediate focus zone 29

Plane of symmetry 31

Impingement regions 33

Input coupling opening 35A

Output coupling opening 35B

Output mirror 37

Second telescope arrangement 39

Optical beam splitting system 41

Second λ/4 waveplate 43

Beam splitter 45

Linear polarization state 47A, 47B

Compressor 49

Dispersive coating 51

Mirror segment 53

Control system 61

Control connections 63

Pressure setting device 65

Pulse duration Δt

Focus diameter d

Rayleigh length Lr

Diameter D

Angle A® 

1. An optical system for increasing contrast of pulsed laser radiation, the optical system comprising: a first polarization setting optical unit for setting an elliptical polarization state of the pulsed laser radiation, a multipass cell having at least two opposing mirrors, wherein the pulsed laser radiation passes the multipass cell with formation of a plurality of intermediate focus zones, and wherein the multipass cell is filled with a gas having an optical nonlinearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation, such that the multipass cell outputs beam portions having differently aligned elliptical polarization states on account of the intensity-dependent rotation, and an optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states.
 2. The optical system as claimed in claim 1, wherein for setting a predetermined rotation angle of the alignment of one of the elliptical polarization states, at least one of the following parameters of the optical system is set or is settable: a gas pressure in the intermediate focus zones, wherein the optical system comprises a gas-filled cell and a pressure setting device for setting the gas pressure in the gas-filled cell, a dispersion present in the multipass cell, an ellipticity of the elliptical polarization state of the pulsed laser radiation, a number of intermediate focus zones in the multipass cell, focus diameters in the intermediate focus zones, and Rayleigh lengths of the intermediate focus zones.
 3. The optical system as claimed in claim 2, wherein the predetermined rotation angle of the alignment of one of the elliptical polarization states is 90°.
 4. The optical system as claimed in claim 1, wherein the first polarization setting optical unit comprises a first waveplate.
 5. The optical system as claimed in claim 4, wherein the first waveplate comprises a λ/4 waveplate and/or a λ/2 waveplate.
 6. The optical system as claimed in claim 1, wherein the optical system furthermore comprises at least one of the following optical components: a pulse duration setting system for setting a pulse duration of primary laser pulses of the pulsed laser radiation, a first optical telescope arrangement configured to image the pulsed laser radiation onto a predefined mode in the multipass cell, the first optical telescope arrangement being disposed downstream of the first polarization setting optical unit, an input coupling mirror for coupling the pulsed laser radiation into the multipass cell, an output coupling mirror for forwarding the pulsed laser radiation emerging from the multipass cell, and a second optical telescope arrangement configured to collimate the pulsed laser radiation emerging from the multipass cell.
 7. The optical system as claimed in claim 1, wherein the multipass cell has a predetermined or settable number of intermediate focus zones, wherein the intermediate focus zones are produced with the opposing mirrors in a resonator set-up with identical radius of curvature of the opposing mirrors, and wherein the intermediate focus zones have substantially an identical diameter and an identical Rayleigh length, wherein the intermediate focus zones are arranged next to one another or partly superposed on one another, wherein the multipass cell is filled with a noble gas, and wherein a same gas pressure is present in each of the intermediate focus zones, wherein the pulsed laser radiation passes through the intermediate focus zones with a substantially constant pulse duration and a substantially constant pulse energy, and/or wherein the pulsed laser radiation passes through the intermediate focus zones with a stepwise nonlinear spectral broadening.
 8. The optical system as claimed in claim 1, wherein at least one of the opposing mirrors of the multipass cell comprises a convex mirror, wherein radii of curvature of the opposing mirrors match, and/or a distance between the opposing mirrors lies in a range of 95% to 105% of a sum of the radii of curvature of the opposing mirrors, and/or wherein at least one of the opposing mirrors comprises a dispersive mirror with a dispersion contribution that compensates for a dispersive contribution of at least one pass of a primary laser pulse of the pulsed laser radiation through the multipass cell, and/or wherein at least one of the opposing mirrors comprises at least one mirror segment, wherein the pulsed laser radiation impinges on the at least one mirror segment at least once during circulation of the pulsed laser radiation through the multipass cell.
 9. The optical system as claimed in claim 1, wherein the multipass cell is configured so that a primary laser pulse of the pulsed laser radiation, for which a contrast is intended to be increased in the optical system, experiences substantially no change in a pulse duration and/or a pulse energy in the intermediate focus zones, and/or wherein the multipass cell is configured as a concentric or confocal resonator.
 10. The optical system as claimed in claim 1, wherein the beam portions having differently aligned elliptical polarization states comprise a useful beam portion with primary laser pulses and a residual beam portion with low-intensity laser radiation, wherein the residual beam portion has a radiation pedestal and/or low-intensity laser pulses preceding the high-intensity laser pulses and/or low-intensity laser pulses succeeding the high-intensity laser pulses.
 11. The optical system as claimed in claim 1, wherein the optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states comprises: a second polarization setting optical unit for returning each of the differently aligned elliptical polarization states to a linear polarization state, wherein the beam portions having differently aligned elliptical polarization states that are output by the multipass cell are converted into a useful beam portion and a residual beam portion having differently aligned linear polarization states, and a beam splitter configured to output the useful beam portion and the residual beam portion on different beam paths.
 12. The optical system as claimed in claim 11, wherein the second polarization setting optical unit comprises a second waveplate, wherein the second waveplate comprises a λ/4 waveplate and/or a λ/2 waveplate.
 13. The optical system as claimed in claim 1, further comprising a control system configured to set at least one of the following parameters, for setting a predetermined rotation angle of the alignment of one of the elliptical polarization states: a pulse duration of primary laser pulses of the pulsed laser radiation, a pulse energy of the primary laser pulses of the pulsed laser radiation, an ellipticity of the elliptical polarization state of the pulsed laser radiation, focus diameters in the intermediate focus zones, Rayleigh lengths of the intermediate focus zones, and a gas pressure of the gas in the intermediate focus zones.
 14. A laser system for emitting pulsed laser radiation, the laser system comprising a laser radiation source configured to output pulsed laser radiation, the pulsed laser radiation comprising primary laser pulses having pulse energies and pulse durations in a range of a few hundred femtoseconds or less, and at least one optical system as claimed in claim 1 for increasing a contrast of the pulsed laser radiation using a nonlinear elliptical polarization rotation in a plurality of intermediate focus zones of a multipass cell.
 15. The laser system as claimed in claim 14 further comprising a pulse duration setting system for setting the pulse duration of the primary laser pulses.
 16. The laser system as claimed in claim 14 further comprising an optical pulse duration compressor system for compensating for a dispersive contribution of the at least one optical system and/or for temporally compressing the primary laser pulses of the pulsed laser radiation as the primary laser pulses have experienced a nonlinear spectral broadening in at least one of the intermediate focus zones.
 17. A method for increasing the contrast of pulsed laser radiation, the method comprising the following steps: setting an elliptical polarization state of the pulsed laser radiation, input coupling the pulsed laser radiation into a multipass cell having at least two mirrors, wherein the at least two mirrors are traversed with formation of a plurality of intermediate focus zones, wherein the multipass cell is filled with a gas having an optical nonlinearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation in the intermediate focus zones, thereby beam portions having differently aligned elliptical polarization states are generated in the multipass cell, output coupling the beam portions having differently aligned elliptical polarizations out of the multipass cell, and separating the output-coupled beam portions into a useful beam portion with primary laser pulses and a residual beam portion with low-intensity laser radiation.
 18. The method as claimed in claim 17, furthermore comprising setting at least one of the following parameters for setting a predetermined rotation angle of the alignment of one of the elliptical polarization states in the multipass cell, a gas pressure in the intermediate focus zones, r a dispersion of the primary laser pulses that has accumulated in the multipass cella pulse duration of the primary laser pulses of the pulsed laser radiation, a pulse energy of the primary laser pulses of the pulsed laser radiation, an ellipticity of the elliptical polarization state of the pulsed laser radiation, focus diameters in the intermediate focus zones, and Rayleigh lengths of the intermediate focus zones.
 19. The method as claimed in claim 18, wherein the predetermined rotation angle of the alignment of one of the elliptical polarization states is 90°.
 20. The method as claimed in claim 17, wherein a pulse spectrum of the pulsed laser radiation assigned to the primary pulses broadens from intermediate focus zone to intermediate focus zone, on account of a nonlinear spectral broadening in the multipass cell, while a contrast is increased simultaneously. 